Ecology of the Opossum on a Natural Area in Northeastern Kansas.

by Henry S. Fitch and Lewis L. Sandidge.

On the 590-acre University of Kansas Natural History Reservation where our study was made, the opossum, _Didelphis marsupialis virginiana_ Kerr, is the largest predatory animal having a permanently resident population. The coyote, racc.o.o.n and red fox also occur on the area but each ranges widely, beyond the Reservation boundaries. With the pa.s.sing nearly a century ago of the larger animals of the original fauna, the buffalo, elk, deer, antelope, wild turkey, gray wolf and others, lesser herbivores and carnivores including the opossum and animals of similar size fell heir to their key positions of predominance at the peak of the food pyramid. These smaller animals, however, exert less powerful effects in controlling the general aspect of the biotic community, and affect it in different directions. The over-all ecology is greatly altered. The flora and fauna both are undergoing successional changes which will continue for a long time and probably will culminate in a biotic community much different from the original climax.

The opossum plays an important part in this process of change; being relatively large, numerous, and of omnivorous habits, it variously influences, directly and indirectly, the populations of its plant and animal a.s.sociates, through a complex web of interrelationships.

Several excellent field- and laboratory-studies of the opossum have been published (Hartman, 1928, 1952; Lay, 1942; Reynolds, 1945; Wiseman and Hendrickson, 1950) and the life history of this remarkable marsupial is already well known. The purpose of our study, therefore, was to gain a better understanding of the ecological relationships of the opossum in the particular region represented by the study area. To accomplish this, we gathered data concerning the animal's responses to climate and varying weather conditions; its annual cycle of breeding, growth and activity, movements, princ.i.p.al food sources, numbers, population turnover, and natural enemies. Although we did gain a somewhat better understanding of the opossum's ecology, results are remarkably meager in proportion to the large amount of time expended.

The hours of work daily in setting and tending a line of live-traps ordinarily were rewarded with only a few records, sometimes none.

Comparable time and effort directed to the study of smaller and more abundant kinds of animals has been far more productive of data. Field work was carried on in parts of 1949, 1950, 1951 and 1952.

[Ill.u.s.tration: FIGURE 1. Map of the University of Kansas Natural History Reservation showing locations where opossums were live-trapped.]

Because opossums are nocturnal and rarely seen in the course of their regular activities, the present study is based mainly on information gained by live-trapping them. Several different sizes of traps of the type described by Fitch (1951) were used. The most successful were 2' 8" 8" in dimensions although many of the larger ones were also used. They were constructed of hardware cloth having a half-inch mesh.

Live-trapping was begun in October 1949 by Fitch with a line of about a dozen traps. In the following month Sandidge joined in the field work. The trapping was continued throughout the winter and spring of 1949-1950 and was resumed the following fall and more traps were added from time to time until a maximum line of approximately 60 was attained. Sandidge's partic.i.p.ation ended in December, 1950. The live-trapping was continued on a reduced scale by Fitch through the winter and spring of 1951 and some was done sporadically in the fall, winter and spring of 1951 to 1952.

Traps were baited with a variety of foods such as carca.s.ses of small vertebrates, meat sc.r.a.ps, canned dog food, ground horse meat and bacon grease. At each capture, s.e.x, weight, and individual formula of the opossum, based on toe-clipping and ear-clipping (Fitch, 1952), were recorded. Also recorded was the exact site of capture as located in one of 84 divisions of the Reservation and estimated in feet from some named landmark. Notes on breeding condition, pelage, injuries, parasites and general appearance were also taken at the time of capture. For opossums caught in 1951 and 1952, the hind foot measurement was recorded.

Often, attempt was made to follow the released opossum to determine the direction and distance of its homeward travel but this was difficult because of brushy terrain and secretive habits of the animal. An opossum being followed would almost invariably take refuge in a tree if it caught sight of the observer. Other information regarding the animal's habits was obtained from tracks in snow or soft soil and from the distribution and contents of scats. Carca.s.ses of opossums which had fallen victim to predators were found on a few occasions and in some instances clues as to the ident.i.ty of the predator were obtained. One hundred and seventeen opossums were live-trapped and handled a total of 276 times. Six of these were dead when first found in the traps. The remaining 111 were marked and released. In addition, 207 pouch-young carried by adult females were recorded and 115 of these were individually marked by toe-clipping.

Some of the opossums that were marked while in the mother's pouch were subsequently recaptured when they were well-grown, independent young, or adults, affording information on growth and dispersal.

HABITAT

The habitats of the Reservation have been described briefly by Fitch (1952) and by Leonard and Goble (1952). More than half the area consists of steep wooded slopes with mixed second growth forest, consisting of elm, hickory, oak, walnut, ash, honey locust, hackberry and osage orange, in about that order of abundance, with thickets of blackberry, crabapple, wild plum and grape. Fallow fields and pastures of the upland and valley floors alternate with the woodland.

The varied habitat provides numerous different food sources. Along the edges of the hilltops there is a nearly continuous limestone outcrop with a lower outcrop paralleling it. These rock ledges, well distributed throughout the area, provide an abundance of den sites and most of the opossums definitely trailed to a home base were found to be utilizing dens in the rock ledges. Two small creeks on the area have some water for most of the year. As compared with wooded bottomland of larger stream courses in Douglas County and those counties adjoining it, the Reservation area probably supports a relatively low population density of opossums. "Sign" has been found in much greater abundance in near-by areas supporting a heavier woodland.

Every part of the Reservation is used by opossums, but their activity is concentrated in the woodland, and all dens found were in woodland.

Most parts of the fields are within 100 yards of the edge of the woodland and no point is more than 700 feet from the edge. Most of the opossums' foraging in fields was concentrated along the edge; otherwise they tended to follow creeks and gullies and they follow well worn trails more often than they do in the woods. Within the woodland, activity tended to be concentrated along the small streams, and along the rock ledges where den sites were plentiful. Throughout the annual cycle, and from year to year, there were minor shifts in areas of concentrated activity depending on seasonal changes in food sources such as thickets of wild plum, crabapple, blackberry and grape, with fruits ripening at slightly different times of year. The areas adjoining the Reservation offer somewhat similar habitat conditions, part woodland, part pasture land and some cultivated fields with corn or other crops which provide food sources for the opossum.

Under original conditions the area that is now the Reservation probably was marginal habitat for opossums, consisting mainly of open gra.s.sland with trees in small and scattered clumps, if indeed they were present at all. There has been steady encroachment of shrubs and trees, originally chiefly confined to near-by bottomlands such as those of the Kaw and Wakarusa valleys. Concurrently, the original hardwood forest of the bottomlands has mostly disappeared, and the land has been taken over for intensive agricultural use. The new upland forest provides a habitat different in many respects from the original bottomland forest. The species composition, in trees and other plants, is somewhat different, with more xeric types, especially on steep south slopes. Logs and large old hollow trees are scarce. The lack of such potential den sites is compensated for by the abundance of holes and crevices along hilltop rock ledges.

BEHAVIOR

Undisturbed opossums were seen in the course of their normal activities on only a few occasions, and behavior is known to us mainly from the sign and from observations made on those that were live-trapped. Ordinarily those taken in live-traps were found curled up in deep sleep from which they did not arouse until touched or until the trap was moved or jarred. Reactions to humans varied greatly in individuals and was not necessarily correlated with age or s.e.x. Adult males were uniformly hostile to the trapper and reacted with harsh, low growls, with back arched and hair bristling. Although many adult females and young of both s.e.xes were similarly hostile in behavior, others were not. Some cowered silently in the trap. Others showed hardly any uneasiness. A small proportion of them feigned death when handled or even before they were touched. Feigning was especially frequent in response to clipping of toes and ears when the animal was marked. In some that were handled, the feigning reaction was weak or incomplete, the animal arising almost immediately after collapsing or beginning to collapse in the feint.

Those that feigned death usually maintained the deception for not more than two or three minutes after a person had moved away out of sight.

The opossum first raised its head and sniffed, listened, and looked about cautiously for a short time, with body and limbs still relaxed in the feigning posture. Failing to detect any sign of danger, it gradually shifted to a sitting position, and then to a standing one, from which it began moving away with many short pauses at first, and then more rapidly.

Upon being released, some opossums scrambled for shelter immediately; others stood their ground defiantly with back arched, hair bristling and fangs bared. One that was put on the defensive would usually maintain its stance for less than a minute if not further disturbed by movements of the trapper. It would then slowly turn its head and begin walking away with deliberate gliding movements, often pausing abruptly in the middle of its stride with one or two feet off the ground in a pose reminiscent of that of a bird dog making its "point." After moving away a few yards, it would gradually accelerate its pace in a scramble for shelter, but an occasional individual moved away unhurriedly, even foraging as it went.

[Ill.u.s.tration: FIGURE 2. Half-mile-square area on Reservation, showing dates and successive sites of capture for two subadult male opossums; one opossum on upper half of map and other opossum on lower half. Arrows from circles show courses taken by released opossums that were followed to dens (crosses).]

On the few occasions when opossums were seen at night, their relative alertness and speed of movement contrasted with the sluggishness and seeming stupidity of those observed in daylight. Several were seen on roads in the beam of automobile headlights. These were quick to escape, running into thick roadside vegetation or woods to elude pursuit. Others were found in woodland, with the aid of a powerful flashlight as the investigator moved about on foot. They did not permit close approach, and escaped by running. One hid in a blackberry thicket. Several that were chased climbed trees when hard pressed. One that was overtaken, and others that were shaken out of trees and caught, showed fight, standing on the defensive, and slashing at the pursuer with a rapidity and vigor never encountered in those removed from traps in the daytime.

[Ill.u.s.tration: FIGURE 3. Half-mile-square area on Reservation, showing dates and successive sites of capture of an old adult male in upper half of map and an adult female in lower half.]

Nocturnal tendencies of the opossum were emphasized by the infrequency with which undisturbed individuals were seen in the daytime. In more than a thousand days of field work on the Reservation, opossums were found out on only four occasions. These occasional daytime forays seem to occur almost always in animals driven by hunger on winter days, when the temperature has suddenly risen after periods of severely cold weather that have imposed inactivity and fasting.

MOVEMENTS

Earlier field studies of the opossum have produced somewhat conflicting evidence and conclusions regarding the extent and manner of the opossum's travels. Lay (1942:158) live-trapped and marked 117 opossums on an 86-acre study area in eastern Texas over a two-year period and caught 29 of them at three or more different trapping stations. He found that "The average minimum area between the stations in these 29 home ranges was 11.5 acres. The mean of the greatest distances traveled between stations was 1460 feet, which would form a theoretical circle of 38.4 acres.... Separate individual territories are not important to opossums as home ranges overlapped in every instance." Reynolds, in central Missouri, concluded that: "The subsequent recovery of only 5 of 68 released animals, the reported capture of one individual 7 miles from the point of release nine months later, and the rapid repopulation of an area devoid of opossums at the close of the hunting season indicate that most opossums are nomadic." In southeastern Iowa, Wisemann and Hendrickson (1950:336) found that: "Recaptures, in 1942, of three opossums tagged in 1941 indicated a yearly mobility of one-fourth mile; four tagged in 1942 were recaptured within one-half mile from sites of tagging."

Opossums, like other animals, obviously make various types of movements. Ordinarily one tends to keep within a relatively small area that is familiar to it and that satisfies all its ecological requirements. This const.i.tutes its home range. Many other animals, including various mammals, are characterized by territoriality; individuals, pairs or groups occupy definite areas, defended as territories, to the exclusion of other members of their species. Like Lay (_loc. cit._) we found no evidence of territoriality in the opossum. In general, opossums are unsocial but not intolerant in their behavior. In the present study numerous individuals of both s.e.xes and various sizes and ages were found to be occupying the same area simultaneously, with overlapping but no exact correspondence in home ranges. Occasionally two or more opossums may use the same den, but each goes its own way on its foraging and it seems that no sociability is involved.

On many occasions opossums were tracked in soft snow or mud which retained footprints. Under conditions prevailing locally, it was difficult to follow such a trail for any great distance but trailing did divulge information concerning the type of route followed and the method of foraging. Opossums were found to have little inclination to follow beaten trails, either their own or those of other animals. A foraging opossum moved about in an extremely circuitous and erratic route, seldom taking more than a few steps without a change of direction, and frequently crossing its own course in a series of loops, some only a few feet or a few inches in diameter. In moving about, it is guided partly by the tactile and olfactory stimuli of objects on or beneath the ground surface which are potential food sources. Foraging consists of a succession of tests of such objects, as the animal moves from one to another. Opossums may habitually follow intermittent creeks or gullies or even roads when these provide better foraging than does the adjoining habitat. Metamorphosing amphibians may provide such a food source along a creek and the supply of crushed insects or other small animals along a road attracts the opossum. Food is found by turning chips and leaves, and by poking and probing in c.h.i.n.ks and crevices with its snout and paws. On a few occasions short, well worn trails made by opossums were found, from dens to near-by feeding areas where grape tangles provided an abundant and readily available food source over periods of weeks. More often, an opossum follows no trail in its search for food, but seems to wander at random within its home range.

[Ill.u.s.tration: FIGURE 4. Quarter-mile-square areas on Reservation showing dates and successive sites of capture of individual opossums; (A) subadult male; (B) subadult male; (C) subadult male; (D) adult female. Arrows from circles show courses that were taken by released opossums that were followed; crosses show location of dens to which they were traced.]

Evidence of the existence and extent of home range was obtained for those opossums that were trapped on several or many occasions. Records of each were usually well scattered over an area hundreds of feet in diameter. Limits of home ranges are not sharply defined and at any time the opossum may extend its range into new areas. It may shift to a new den from which areas beyond its original home range are readily accessible, and may then occupy a new home range overlapping part of the old one. Or, it may make a relatively long shift, to an area entirely distinct from the original home range and well separated from it. That such shifts are frequent was indicated by the brief span of records for most of the opossums live-trapped on the Reservation.

After the first capture and marking an individual was often caught consistently over periods of weeks, only to drop out suddenly either having been eliminated or having moved elsewhere. Of the 111 opossums marked and released, 62 were caught only once and 25 others were recaptured only within a period of one or two months. Relatively few, only 24 (14 males and 10 females), had records extending over more than two months. Many of the opossums trapped were probably at or near the edges of their home ranges which barely overlapped the study area; consequently the chances of recapturing them were poor. Those caught well within the trapping area were much more likely to be recaptured.

Tracking of opossums suggested that having once left the home den, an animal ordinarily did not return until it had finished its nightly foraging, and wandered more or less at random over its home range.

Successive capture sites for any one opossum might be near together or far apart with respect to its over-all range, but on the average, they would be separated by approximately half the breadth of the home range a.s.suming the animal's activity to be evenly distributed over the whole area. Each of twenty-two opossums was caught at only two different trapping stations. For this group, the average distance between stations was 761 feet (657 feet for seven males and 810 feet for 15 females) indicating home ranges of approximately 42 acres in extent.

Each of ten opossums was caught at three different stations; for these the distances between the first and second stations, between the first and third and between the second and third comprise three distinct movement records, and the average of all three probably affords a more reliable figure for the radius of the home range than does the single movement available for each of the 22 animals captured at only two stations. For these average individual movements the mean of this whole group of 10 was 841.5 feet. Each of five opossums was taken at 4 different trapping stations, and for each of these a record of six different movements was available. The average was 1016 feet. For the 37 opossums caught at two, three or four different trapping stations, the mean distance was 817 feet; this is an indication of home ranges of approximately 48 acres in extent. Each of thirteen opossums was caught at five or more trapping stations. The distribution of these stations affords a crude idea of the extent and position of each animal's home range, but ordinarily it might be expected that the area included between capture sites would be less than the animal's actual home range, because relatively few of the sites of capture would be on the margin of the home range. For this group, maximum distances between trapping stations averaged 1954 feet suggesting a home range of nearly 70 acres, larger than that computed for the opossums caught at only two, three, or four stations. However, for those caught at five or more stations, the time involved averaged longer and probably some had altered their ranges to invade new areas. Ranges may have been broadly oval rather than circular so that the maximum diameter measured between stations exceeded somewhat the average range diameter for each animal.

The opossums having home ranges entirely within the study area were those most likely to be caught repeatedly and at different locations, while those with ranges centering near the edge of the area, or outside of it tended to be caught at fewer locations and less frequently. For those animals with ranges partly outside the study area, the captures recorded would represent only one sector of the home range and would tend to be near together, so that many of the radii computed for individual home ranges are too small. Each average figure for home range is perhaps erroneously low for this reason. The error tends to be greatest for those taken at only two locations, and least for those trapped at the greatest number of different locations.

Approximate size of the usual home range is apparent from the several figures although various unknown or unmeasurable factors distort the data. The usual home range of the opossum in the area of the study is in the neighborhood of 50 acres or a little less. With the data available no significant differences in sizes of home ranges are discernible between males and females nor between adults and young of the year. Shifts occur frequently, contributing to population turnover, which may result in almost complete replacement of individuals in the course of a year's time, on an area of less than a square mile.

DISPERSAL OF YOUNG

One hundred and fifteen small young of 14 different litters were marked while still attached to the mother's teats in the pouches.

A fairly high rate of mortality probably is normal in the small dependent young and further mortality probably resulted from the deleterious effects of examining and handling them and the females that carried them. At any rate, 47 of 208 young recorded, were missing at subsequent recaptures of the females, before the young were old enough to become independent. It is almost certain that the actual losses were much higher, because the records for each female cover only part of the period during which young are carried in the pouch.

Fifteen of these marked young of seven different litters were recaptured after periods of months, when they were well grown or adult and the locations of these recaptures afford information concerning the animals' dispersal. Their records are summarized below. Opossums that wandered much more than half a mile or at most three-fourths of a mile from the place of original capture were unlikely to be recaptured, and some originally recorded at sites near the edge of the study area might have moved beyond its boundary with much shorter shifts.

Date of capture and Date of Distance s.e.x marking as pouch young recapture in feet

Female April 14, 1951 September 22, 1951 1870 Female May 6, 1950 February 28, 1952 1800 Female May 14, 1950 December 31, 1950 1750 Female March 28, 1951 January 23, 1952 1700 Female May 11, 1951 November 9, 1951 1700 Female May 11, 1951 March 2, 1952 1450 Female April 2, 1950 October 7, 1950 1160 Female April 14, 1951 May 19, 1952 1100 Male May 11, 1951 February 3, 1952 800 Female May 11, 1951 January 9, 1952 700 Female April 2, 1950 October 3, 1950 700 Female May 6, 1950 April 3, 1951 650 Female March 28, 1951 February 2, 1952 500 Male April 18, 1952 July 6, 1952 120 Female April 2, 1950 April 14, 1951 10

Most of these opossums were recaptured within a year of the time they were marked as small young in the females' pouches, and on the average they had moved a little less than 400 yards. While the s.e.x ratio was equal in the pouch young that were marked, it is noteworthy that all but two of the recaptured opossums were females; and of the two males, one was recaptured early, before it could have had time to wander far.

The young males, after becoming independent must tend to wander much more widely, and to settle in new areas far removed from the mother's home range. It is unlikely that this dispersal of the young males is motivated either by rivalry and intolerance of larger males or by s.e.xual drive. The dispersal occurs in late summer when there is no breeding activity, and when food is present in greatest abundance and variety.